The accumulation of fireside deposits on the internal heating surfaces of boilers drastically reduces their thermal efficiency and, if not removed, requires periodic shutdowns of the boiler for manual cleaning. The principal means for removing fireside deposit accumulation in boilers is a cleaning device known as a sootblower which directs jets of fluid cleaning agents, such as steam, air and/or water, against the internal surfaces of the boiler. The cleaning effectiveness of a sootblower depends to a great degree on the nozzle design which controls the mass flow, exit speed and the jet decay characteristics of the exiting jets.
The sootblower nozzle design most commonly used today is based on the de Laval design comprising convergent and divergent flow sections which form a venturi. The pressure of the cleaning agent decreases as it passes through the convergent segment of the nozzle, attaining the speed of sound at the throat of the nozzle. The pressure of the fluid cleaning agent then decreases further through the divergent section, expanding and accelerating from the nozzle throat to the nozzle exit and thereby typically exceeding the speed of sound as the cleaning agent exits. The pressure drop over the divergent section is controlled by the designed geometry of that section.
The cleaning potential of the jet emitted from a nozzle is commonly measured in terms of the jet's Peak Impact Pressure (PIP). The maximum PIP is delivered by nozzles where the pressure of the exiting jet equals the ambient pressure resulting in a "fully expanded" jet. Nozzles which do not allow the pressure of the exiting jet to reach the ambient pressure result in "under expanded" jets. In the case of under expanded jets, the pressure of the exiting jet is higher than the ambient pressure so the exiting jet must finish expanding outside the nozzle causing a series of expansion and contraction waves called "shock waves." These "shock waves" convert a substantial part of the kinetic energy of the jet stream into internal energy, thereby markedly reducing the PIP.
A "full expansion" nozzle is achieved by designing the nozzle with a specific ratio between the area of the nozzle's exit to the area of the nozzles's throat. The ratio is determined by the particular nozzle inlet pressure. In practice, this means the length of the divergent segment of the nozzle, L.sub.n, needs to be long enough to allow the full expansion and corresponding drop in pressure of the fluid cleaning agent down to the ambient pressure at the nozzle's exit. However, for most practical sootblower applications, the available space for a nozzle limits the use of conventional full expansion nozzles because their nozzle length is too great as shown in Table 1.
TABLE I ______________________________________ Conventional Full Nominal Throat Nozzle Expansion Size Area Flow Rate Length Nozzle Length (in.) (in..sup.2) (lbs/sec.) (in.) (in.) ______________________________________ 7/8 0.601 2.24 1.63 3.45 1 0.785 2.93 1.63 3.86 1 1/8 0.994 3.71 1.63 4.95 ______________________________________ *For 300 psi inlet pressure and 600.degree. F. superheated steam.
Consequently, only the shorter under expanded nozzles are used. These circumstances are most apparent with so called long retractable sootblowers, such as the one disclosed in European Patent No. 159,128. The sootblower of the '128 patent uses a lance tube typically having a plurality of nozzles at its working end which are generally positioned opposite to each other, with aligned center axes or slightly staggered center axes in order to offset the jet reaction forces, as seen in FIG. 2 of the '128 patent.
In a conventional sootblower, nearly the whole nozzle member is disposed internally in the nozzle head so that it may be inserted into the boiler furnace through a tight wall box which opens into the boiler wall. Typically the nozzle head outer diameter in conventional sootblowers is 3.5 inches, and the inside diameter is 3.0 inches. Therefore, the length of the sidewise pointed nozzle body cannot exceed approximately 1.63 inches. To obtain the required mass fluid flow for adequate cleaning, a conventional full expansion nozzle requires a length between 3 and 5 inches based on a common inlet pressure of 300 psi and superheated steam of 600.degree. F. Consequently, a conventional sootblower nozzle head is not able to house nozzles capable of generating fully expanded jets.